Registration Surrogate (RS) is a technique that enables an operator to register GSM subscribers or ISDN subscribers in an Internet Protocol Multimedia Subsystem (IMS) network. The registered IMS subscribers can use GSM access, but have their services reside in the IMS network. The RS facilitates triggering of one or more IMS services when a call to/from such a GSM subscriber is routed from a GSM network to an IMS network. The routing of a call from GSM to IMS may be performed using the well known Customized Application of Mobile Network Enhanced Logic (CAMEL) “Route Home” service. The routing of a GSM call through IMS, for the purpose of executing one or more IMS services in the IMS network, is known as “GSM-IMS overlay”.
One example of an IMS service for which it is desirable to be accessible in the above mentioned manner is IP-Centrex. IP-Centrex is an IMS service offering Wireless Office features. One of the aspects of IP-Centrex is the possibility to access an IMS service with GSM terminals, as well as with Session Initiation Protocol (SIP) phones. Such IMS service is commonly referred to as Multi Access IMS service. The remainder of the description takes IP-Centrex as example IMS service. The invention is, however, equally applicable to any other IMS service that may be accessed through GSM, such as Call Completion or Incoming Call Screening. Furthermore, the invention is equally applicable to calls in fixed telecommunication networks, as in mobile networks such as GSM networks or UMTS networks.
When applying CAMEL for the overlay between GSM and IMS, the called party number (CdPN), as dialled by the calling subscriber in the case of calls originating in the GSM network, can not always be maintained in the ISDN User Part (ISUP) signalling between a GSM network and an IMS network. This has mainly three reasons.
Firstly, the CdPN that is carried in the ISUP Initial Address Message (IAM) is limited to 15 address digits, by ITU-T recommendation E.164. But 15 digits may not be sufficient to address a Media Gateway Control Function (MGCF) and also includes the original dialled number, especially when the GSM call is established from abroad or from a network of a different provider. Secondly, the number frame of the CdPN in ISUP contains a Nature of Address (NoA). When an MGCF address, required for routing the call to the MGCF, is added to the CdPN, the NoA in the frame of the ISUP CdPN has to be set to national or international, depending on whether the GSM call is established in the home country or from abroad or from a different provider. As a result, the NoA of the original dialled number gets lost. A third reason is that the user may include operator-specific service selection codes (* and #) in the dialled number. These codes can not be conveyed, through ISUP, to the IMS service.
Also, when ISUP signalling traverses (inter)national network boundary, the Calling party number (CgPN) may get corrupted or lost. The CgPN is however needed to identify the calling subscriber and to trigger the IMS service. Hence, when the CgPN is not available or when it is corrupted, there is no IMS triggering possible and hence, the call can not be established through IMS overlay.
Finally, when the IMS service is triggered, vital information such as Location Information is not available to the IMS service.